Hidden Molten Channel Beneath Earth Discovered with a Blast

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The bottom of one of Earth's rocky plates has been visualized in
fine detail using sound waves from dynamite exploded deep
underground, revealing a once-hidden channel of molten rock.

While the images are impressive in their own right, the findings
could also provide insight into a long-standing question about
the mechanics of plate
tectonics, the theory that Earth's outer shell is divided
into "plates" that slowly move over the mantle (the molten-rock
layer above the planet's core) over millions of years, said study
co-author Tim Stern, a geologist at Victoria University of
Wellington in New Zealand.

In fact, the huge channel may create a soft spot that ultimately
explains at least one rocky plate's movements.

The team had placed about 0.5 tons of dynamite in several
steel-encased bore holes along the subduction zone. When the
dynamite exploded, it sent powerful sound waves into the holes.
By measuring the reflected waves, the team could then visualize
the top of the plate, about 12 to 18 miles (20 to 30 kilometers)
beneath the surface.

"Much to our surprise, we got more echoes from much deeper,"
Stern told Live Science.

By looking more closely, the team concluded the echoes had come
from the bottom of the plate, about 45 miles (73 km) below the
top of the plate.

The accidental discovery provided a much more detailed view of a
tectonic plate's base than past methods, such as measuring
seismic waves from earthquakes, Stern said.

In addition, the team found that near the bottom of the plate,
the sound waves moved a little slower before being reflected. The
researchers speculated that water or a tiny bit of melt from the
Earth's
mantle was pooling at the base of the plate, creating a
narrow channel in the transition zone between the base of the
plate and the upper portion of the mantle. The movement of the
plate was further trapping this melt, creating a soft spot and
helping the plate glide atop the mantle more easily.

The findings could have implications for one of the big
unanswered questions in geology: how the tectonic
plates that make up Earth's outer shell move. One theory
holds that currents of heat rise from the deep mantle, pushing
upward on the continental plates and creating drag as these
currents of heat cycle downward again. But the presence of a soft
spot at the base of the plate makes it unlikely that enough drag
force could be applied there to move the plates, Stern said.

Instead, the current findings lend greater weight to another
theory, in which the heavier tectonic plates sink at their edges
into the mantle at subduction zones, Stern said. That sinking
creates a momentum that then tugs the rest of the plate with
them.

"It's a cool or neat result in its own right, the ability to
image such a fine structure at such a depth," said Simon
Klemperer, a geologist at Stanford University, who was not
involved in the study.

The findings also suggest that drag force from mantle convection
is probably less important in the movement of tectonic plates
than previously thought, and that the gravitational effects at
the plates' edges may play a bigger role than thought, Klemperer
told Live Science.

It's not clear yet if the mysterious channel, the soft spot, is
unique to this particular subduction zone. There is some evidence
for a channel underneath Costa Rica and on the continental plate
that descends beneath Norway. But to show that sinking is more
important in explaining how tectonic plates move, the team would
have to show similar channels around the world at both
continental and oceanic plates, Klemperer said.